The present invention relates to an apparatus which handles optical information readably recorded in, for example, an optical disk, optical card, magneto-optical disk and so forth (collectively referred to as "optical information medium"). More particularly, the present invention is concerned with an optical information reading or retrieval apparatus for reading optical information recorded in such an optical information medium.
An optical information reading apparatus is disclosed in an article entitled "An Integrated-Optic Disc Pickup Device" on the Journal of the Institute of Electronics, Information and Engineering, vol. J69-C, No. 5, pp 609-615 (May 1986). The light beam from a source is made to be directed to a slab waveguide, and is then made to be emitted out of the waveguide plane by the acion of a diffraction grating coupler, so as to be focused on the optical information medium.
This known apparatus, however, suffers from a problem in that a large aberration tends to be caused by fluctuation in the wavelength of the beam from te source. The aberration attributable to fluctuation in the wavelength results in a fluctuation in the focal position of the beam which is irradiated on the surface of the information medium for the purpose of reading information recorded in the medium. The fluctuation in the focal position impedes stable reading of the information, thus causing a serious problem. The fluctuation in the focal position can be roughly divided into two types: namely,
(a) fluctuation of focal position in the direction of the optical axis (change in the focal distance) and
(b) fluctuation of focal position in a direction perpendicular to the optical axis.
The fluctuation in the direction of optical axis causes defocusing, while the fluctuation in the direction perpendicular to the optical axis causes track jump or bit omission.